DISTRIBUTION LAW (LIMITATIONS AND APPLICATIONS)

PROBLEM 1.

A solid X is added to a mixture of benzene and water.

After shaking well and allowing to stand, 10 ml of the

benzene layer was found to contain 0.13 g of X and 100

ml of water layer contained 0.22 g of X. Calculate the

value of distribution coefficient.

SOLUTION 1 -

Concentration of X in benzene (Cb) = 0.13 / 10 = 0.013g ml–1

Concentration of X in water (Cw) = 0.22 / 100 = 0.0022g ml–1

According to Distribution law :

Cb/ Cw = 0.013 / 0.0022 = =5.9

SOLUBILITIES AND DISTRIBUTION LAW

When a solute is shaken with two non-miscible solvents, at equilibrium both the

solvents are saturated with the solute. Since the solubility also represents concentration,

we can write the distribution law as

C1/C2 = S1/S2 = KD

where S1 and S2 are the solubilities of the solute in the two solvents.

Hence knowing the value of the Distribution coefficient (KD) and the solubility of

solute in one of the solvents, the solubility of solute in the second solvent can be

calculated.

APPLICATION OF DISTRIBUTION LAW

There are numerous applications of distribution law in the

laboratory as well as in industry

(1) Solvent Extraction-

This is the process used for the separation of organic

substances from aqueous solutions. The aqueous solution is

shaken with an immiscible organic solvent such as ether (or

benzene) in a separatory funnel. The distribution ratio being in

favour of ether, most of the organic substance passes into the

ethereal layer. The ethereal layer is separated and ether distilled

off. Organic substance is left behind.

2) Partition Chromatography

A paste of the mixture is applied at the top of a column of

silica soaked in water. Another immiscible solvent ( hexane)

is allowed to flow down the column. Each component of the

mixture is partitioned between the stationary liquid phase

(water) and the mobile liquid phase (hexane). The various

components of the mixture are extracted by hexane in order

of their distribution coefficients.

(3) Desilverization of Lead (Parke’s Process)

When molten zinc is added to molten lead containing silver

(argentiferous lead), zinc and lead form immiscible layers

and silver is distributed between them. Since the distribution

ratio is about 300 in favour of zinc at 800º C, most of silver

passes into the zinc layer. On cooling the zinc layer, an alloy

of silver and zinc separates. The Ag-Zn alloy is distilled in a

retort when zinc passes over leaving silver behind. The lead

layer still contains unextracted silver. This is treated with fresh

quantities of molten zinc to recover most of the silver

(4) Confirmatory Test for Bromide and Iodide

The salt solution is treated with chlorine water. Small quantity

of bromine or iodine is thus liberated. The solution is then

shaken with chloroform. On standing chloroform forms the

lower layer. The free bromine or iodine being more soluble in

chloroform concentrates into the lower layer, making it red for

bromine and violet for iodine.

5)Determination of Association

When a substance is associated (or polymerized) in solvent A

and exists as simple molecules in solvent B, the Distribution law

is modified as

n√Ca/Cb = k

when n is the number of molecules which combine to form an

associated molecule.

(6) Determination of Dissociation

Suppose a substance X is dissociated in aqueous layer and exists as single

molecules in ether.

If x is the degree of dissociation (or ionisation), the distribution law is modified

as

C1 /C2 )(1-x) = K

where C1 = concentration of X in benzene

C2 = concentration of X in aqueous layer

The value of x can be determined from conductivity measurements, while C1

and C2 are found experimentally. Thus the value of K can be calculated. Using

this value of K, the value of x for any other concentrations of X can be

determined.

(7) Determination of Solubility

Suppose the solubility of iodine in benzene is to be determined.

Iodine is shaken with water and benzene. At equilibrium

concentrations of iodine in benzene (Cb) and water (Cw) are

found experimentally and the value of distribution coefficient

calculated.

Cb / Cw = Kd

Sb/ Sw = Kd

where Sb = solubility in benzene; and Sw = solubility in water.

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